This invention relates in general to a vacuum evaporation system and more specifically, to apparatus and processes for vacuum evaporating vaporizable materials.
In the art of electrophotography an electrophotographic plate comprising a photoconductive layer on a conductive layer is imaged by first uniformly electrostatically charging the imaging surface of the photoconductive layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive layers. Numerous different types of electrophotographic imaging members for xerography, i.e. photoreceptors, can be used in the electrophotographic imaging process. Such electrophotographic imaging members may include inorganic materials, organic materials, and mixtures thereof. Electrophotographic imaging members may comprise contiguous layers in which at least one of the layers performs a charge generation function and another layer forms a charge carrier transport function or may comprise a single layer which performs both the generation and transport functions. The electrophotographic plate may be in the form of a plate, drum, flexible photoreceptor web, sheet, flexible belt and the like.
The photoconductive layer or layers may be formed of various materials. If the photoconductive materials are vaporizable and do not decompose at vaporizing temperatures, they can often be deposited by vacuum deposition. Similarly, vaporizable materials may be vacuum deposited for various other applications such as solar cells, metallic layers for decorative packaging, capacitors, optical coatings on glass and the like.
Electrophotographic imaging members based on amorphous selenium have been modified to improve panchromatic response, increase speed and to improve color copyability. These devices are typically based on alloys of selenium with tellurium and/or arsenic. These selenium electrophotographic imaging members may be fabricated as single layer devices comprising a selenium-tellurium, selenium-arsenic or selenium-tellurium-arsenic alloy layer which performs both charge generation and charge transport functions. The selenium electrophotographic imaging members may also comprise multiple layers such as, for example, a selenium alloy transport layer and a contiguous selenium alloy generator layer.
A common technique for manufacturing photoreceptor plates involves vacuum deposition of a selenium alloy to form an electrophotographic imaging layer on a substrate. Tellurium is incorporated as an additive for the purpose of enhancing the spectral sensitivity of the photoconductor. Arsenic is incorporated as an additive for the purpose of improving wear characteristics, passivating against crystallization and improving electrical properties. Typically, the tellurium addition is incorporated as a thin selenium-tellurium alloy layer deposited over a selenium alloy base layer in order to achieve the benefits of the photogeneration characteristics of SeTe with the beneficial transport characteristics of SeAs alloys. Fractionation of the tellurium and/or arsenic composition during evaporation results in a concentration gradient in the deposited selenium alloy layer during vacuum evaporation. Thus, the term "fractionation" is used to describe inhomogeneities in the stoichiometry of vacuum deposited alloy thin films. Fractionation occurs as a result of differences in the partial vapor pressure of the molecular species present over the solid and liquid phases of binary, ternary and other multicomponent alloys. Alloy fractionation is a generic problem with chalcogenide alloys. A key element in the fabrication of doped photoreceptors is the control of fractionation of alloy components such as tellurium and/or arsenic during the evaporation of selenium alloy layers. Tellurium and/or arsenic fractionation control is particularly important because the local tellurium and/or arsenic concentration at the extreme top surface of the structure, denoted as top surface tellurium (TST) or top surface arsenic (TSA), directly affects xerographic sensitivity, charge acceptance, dark discharge, copy quality, photoreceptor wear and crystallization resistance. In single layer low arsenic selenium alloy photoreceptors, arsenic enrichment at the top surface due to fractionation can also cause severe reticulation of the evaporated film. In two layer or multilayer photoreceptors where low arsenic alloys may be incorporated as a base or transport layer, arsenic enrichment at the interface with the layer above can lead to severe residual cycle up problems. In single layer tellurium selenium alloy photoreceptors, tellurium enrichment at the top surface due to fractionation can cause undue sensitivity enhancement, poor charge acceptance and enhancement of dark discharge. In two layer or multilayer photoreceptors where tellurium alloys may be incorporated as a generator layer, tellurium enrichment at the upper surface of the tellurium alloy layer can result in similar undue sensitivity enhancement, poor charge acceptance, and enhancement of dark discharge.
Another common technique for manufacturing photoreceptors involves vacuum deposition of organic and inorganic pigments to form a thin charge generation layer. This charge generation layer together with a thicker charge transport layer form an electrophotographic imaging layer on a substrate. A typical thickness of the charge generation layer is between about 0.05 micrometer and about 1 micrometer with about 0.1 micrometer to about 0.5 micrometer being preferred. The pigment material may comprise a selenum-tellurium alloy with a high concentration of tellurium for red sensitivity or may comprise an organic pigment such as phthalocyanine, perylene, or other polycyclic pigment that is thermally stable. These organic pigments sublime when heated in the vacuum to temperatures above about 400.degree. C. Because they do not melt and make good thermal contact with the crucible, it is preferable that they are vacuum deposited out of an isothermal source. Furthermore, while these pigments are stable at elevated temperatures in an inert container in a vacuum in the presence of metals and other impurities they may decompose or react partially. Thus, it is preferable that the evaporation source be made out of an inert materials such as quartz.
Two types of techniques are used in thermal evaporation and vacuum deposition of materials. Free evaporation directly from solid surfaces (sometimes referred to as Langmuir evaporation), is approximated by shallow open crucible sources and is the most commonly used technique. This type of free evaporation from open crucible sources promote fractionation of multi-component evaporant materials such as mixtures of selenium with arsenic and/or tellurium. In the other technique, called Knudsen's method, evaporation occurs as effusion from an isothermal enclosure or crucible with a small orifice. The evaporation surface inside the enclosure is large compared with the size of the orifice and maintains an equilibrium pressure inside. The enclosed Knudsen type of source has two advantages: the enclosed source eliminates spatter due to localized vaporization by poorly conducting materials and gives a greater latitude in choosing temperature and pressure conditions that will permit a multicomponent material to be in equilibrium and evaporate congruently. When a multicomponent material is in equilibrium and evaporates congruently, the composition of the deposited coating is constant with time. Many vaporizable materials such as, for example, alloys of selenium, arsenic and/or tellurium can be evaporated congruently under the appropriate conditions. Tube crucibles with a constricted slit approximate a Knudsen cell and facilitate attainment of equilibrium and congruent evaporation of multicomponent materials. The geometry of a tube crucible having a constricted slit also permits easy fabrication and uniform heating by resistance with no cold or hot spots.
Although excellent deposits may be achieved with tube crucibles having constricted slits or slots, loading of evaporants through the narrow slot opening is difficult, slow, tedious, and sometimes, impossible because of the relative size of the particles being loaded and the width of the slot in the Knudsen-type crucible. If the slit opening is widened to facilitate loading of the crucible, the performance of the crucible approaches that of an open crucible. On the other hand, abandonment of the simple tube geometry concept to fabricate compound crucibles with a removable cover to allow loading introduces difficulties in maintaining temperature uniformity within the crucible. It also renders loading more complex (particularly in planetary coating devices), difficult, expensive and time consuming. Further, after one or more coating runs, it may be necessary to clean the crucible of residue as the resulting debris can cause defects to occur in subsequently formed photoreceptor layers.
Generally, because of the importance of maintaining a fixed distance between the crucibles and the substrates to be coated, and because of the massive electrical connections utilized between the electrically conductive crucibles and the power source, the crucibles are normally rigidity mounted in position and removal thereof is difficult and time consuming. Moreover, because the crucibles are normally semipermanently mounted in the vacuum chamber, production is delayed for loading of the crucibles with the evaporant and for cleaning. Further, cleaning of the Knudsen-type crucibles is extremely difficult because of the small slot widths.